A common problem faced by authors of confidential documents is the difficulty of preventing unauthorized persons from gaining access to them. A related problem is breach of copyright of printed texts. These problems have become particularly acute with the commonplace availability of photocopiers as it is a simple matter for a person legitimately possessing a certain document to copy it and pass it on to other, perhaps unauthorized, persons. To date, prevention of such transgressions has been mainly by physical restriction of availability, for example by denying access to all but a trustworthy few, and relying on punitive measures to prevent breach of confidence. As a further deterrent confidential documents are sometimes stamped with the words "SECRET" or "CONFIDENTIAL". Such techniques have little effect and efforts have been made to mark documents in such a way that each copy bears a distinctive feature so that in the event of "leaking" the source of the leak can be identified. One such technique involves subtly changing the text of each copy distributed, for example by introducing spelling mistakes, and recording the details of changes made in the case of each recipient. This technique is cumbersome, requires undesirable alteration of the text, and relies on recovery of the whole or a large part of the text to be effective as a means of identifying the source. Other marking schemes utilize a pattern of special markings which must cover the whole page area if the possibility of erasure is to be avoided. Some document marking schemes rely on subtle variations of character formation, for example by changing the typeface of selected characters. One such system, described in Patent Cooperation treaty International Publication WO 85/05713 of Millet, used small variations in the formation of characters produced by a dot-matrix printer to uniquely mark documents. For example, Millet's system can selectively omit dots of the usual matrix to create deliberately mis-formed characters. This scheme is only effective in a limited range of applications, since the subtle variations are easily lost if the document is blurred by photocopying, particularly if multiple generations of copies are made.
An improved document marking scheme has been published under the Patent Cooperation treaty International Publication number WO 89/07819 of Vogel. This publication discloses a scheme which marks documents by varying the spacing between words. According to this scheme the pattern of space variations conveys marking information which is unique to each copy of a document printed and can be used to trace the source of a particular copy in the case of a leak.
Although the scheme of WO 89/07819 is very useful, the improved marking scheme of the present invention is responsive to the content of a document so that the system of marking is optimized according to the format of the text. For example, where text is in tabular format, spacing between lines is varied for marking rather than spacing between words, thereby maintaining columnar alignment.
One shortcoming of prior-art document marking schemes is that they do not solve the problem of marking unmarked documents copied using a photocopier. For example, there is a strong need for a system which will deter people from copying books in libraries, an act which could infringe copyright.
The present invention is directed to providing an improved document marking scheme which uniquely identifies each of a potentially large number of copies of a document. A further novel and useful feature of the invention is the ability to convey within the marking textual information, such as the name of the intended recipient of each copy. The invention can also be used to encode long passages of text within a document in a way that is not readily visible but can be decoded to reveal the original encoded text. Furthermore the marking scheme of the invention does not significantly detract from the visual quality of the document, and the marking is maintained in spite of photocopying, enlarging or reducing. Another feature of the present invention is that it can be effectively used to mark text of a wide variety of formats, including tabular text.
The invention can also be practiced in the form of a photocopier which uniquely marks copies produced, so that, for example, if a person copies a book in a library, the name of the person making the copy is marked into the copy made. The invention can be realized by a suitable arrangement of conventional digital data processing components. Other advantages and objects of this invention will be more fully appreciated from the description of the invention which follows.